Answer
Carmen replies
When I’m trying to paint something realistically, I always begin with a little bit of research. The more thoroughly I understand something, the more convincingly I can paint it from imagination later on. Ask yourself questions, and ensure that you obtain the answers through research and observation.
Why does skin bruise? What causes the different colours? How does a bruise made by blunt object trauma differ from one caused by muscle sprains? What causes soft versus hard edges? Looking for these answers forces me to slow down and closely observe my reference before I begin painting, preventing me from rushing ahead without a plan.
First, I start with the base skin tone, because this will determine what colours are used for the bruise. Next, I choose colours based on my reference and research, and layer them over the skin. I use Multiply layers when I need a darker value, Soft Light to change hue, and Overlay to bump up the colour intensity. Finally, I add texture.
The discolouration of a bruise is underneath the skin, and this means pores, hairs or other surface markings will still be visible. The texture of the pores is exaggerated in bruises. One of the most common mistakes is making soft, fuzzy bruises on top of the skin.
And that’s pretty much all there is to it! And if you did your research, you may not even need to reach for reference next time you need to rough up your characters.